The best is yet to come: reward prediction in the brain

Jonah Lehrer has written an excellent piece for the latest issue of Seed Magazine on the work of neuroscientist Read Montague who’s been discovering the essential function of dopamine in predicting rewards.

Reward prediction is the process where dopamine neurons fire when a reward is expected and also seem to code the amount of error between the prediction and what actually happens. Importantly, the process seems to be accurately described by an algorithm that was already used in computer science.

This has been an area of intense interest over the last decade as it ties together neurobiology, learning, motivation, mathematics and can be demonstrated in a variety of simple lab-based tasks. The fact that dopamine has been linked to numerous disorders in the past makes it a popular paradigm in which to understand psychiatric symptoms.

The Seed article looks at the work of Read Montague who has been studying the process and has been using ingenious methods to look at the role of this system in social reasoning.

In recent years Montague has shown how this basic computational mechanism is a fundamental feature of the human mind. Consider a paper on the neural foundations of trust, recently published in Science. The experiment was born out of Montague‚Äôs frustration with the limitations of conventional fMRI. ‚ÄúThe most unrealistic element [of fMRI experiments] is that we could only study the brain by itself,‚Äù Montague says. ‚ÄúBut when are brains ever by themselves?‚Äù And so Montague pioneered a technique known as hyper-scanning, allowing subjects in different fMRI machines to interact in real time. His experiment revolved around a simple economic game in which getting the maximum reward required the strangers to trust one another. However, if one of the players grew especially selfish, he or she could always steal from the pot and erase the tenuous bond of trust. By monitoring the players‚Äô brains, Montague was able to predict whether or not someone would steal money several seconds before the theft actually occurred. The secret was a cortical area known as the caudate nucleus, which closely tracked the payouts from the other player. Montague noticed that whenever the caudate exhibited reduced activity, trust tended to break down.

One thing I notice a little of in the quotes from Montague, which is incredibly common in discussion of dopamine and reward, is a kind of ‘reward system dogma’.

Reward is usually linked to the function of the striatum and nucleus accumbens and the dogma goes something like this: “no matter what is happening when the nucleus accumbens or striatum is activated, something about the activity is rewarding”.

I was interesting to read a recent study comparing brain activation in people with ‘normal’ and ‘complicated’ (i.e. extreme) grief in response to viewing pictures of their deceased relative.

The study found additional nucleus accumbens activation in people with complicated grief and suggested that this reflects the fact they find the thoughts of them more rewarding. This is despite the fact that the nucleus accumbens has also been found to also represent salience – i.e. how likely something is to grab our attention.

It’s probably also worth mentioning that there may be some serious problems with the elegant reward prediction theory of dopamine which are were outline in a 2006 paper in Nature Reviews Neuroscience and summarised by the excellent Developing Intelligence.

The Seed is generally an excellent read though and covers an important finding and some innovative new ideas. I especially like the fMRI machines linked in parallel, like multi-player arcade machines.